An arms race between microbe and immune response determines whether the invader will be eliminated or establish a persistent infection. In the setting of chronic infection, a balance exists between immune activation, to control viral replication, and immune suppression, to control immunopathology. In lymphocytic choriomeningitis virus (LCMV) clone 13, a mouse model of chronic infection, multiple modulators of the immune response, including IL-10, PD-1, and TGF-β, have been implicated in the phenomenon of T cell exhaustion. Upon continued antigenic stimulation, CD8+ T cells lose their effector function, and are selectively deleted, resulting in failure to achieve clearance. Treatment with IL-7 has recently been shown to drive T cell expansion and function by decreasing SOCS3 levels, allowing for virus clearance in the absence of collateral damage (see figure, Enhancing the immune response to a chronic viral infection).

In the battle between immune system and microbe, the winner persists in the form of either a memory T cell or a chronic infection. During initial infection, a classical acute response is mounted. This includes expansion of CD8+ T cells, and secretion of IL-2 and IFN γ. CD8+ T cells acquire cytotoxic effector functions and tissue trafficking ability and with help from other components of the immune system control the viral infection. After the acute response has peaked, more than 90% of effector CD8+ T cells die and the remaining cells differentiate into memory cells. Memory cells are maintained via cytokine induced homeostatic proliferation.

To maintain a state of chronic infection, the immune response must be dampened to prevent excessive collateral damage. Understanding this infection state is important with more than 500 million people worldwide chronically infected with HIV, Hepatitis C virus (HCV), and Hepatitis B virus (HBV). Understanding this persistent state of infection will aid in discovery of novel treatment strategies.

Viruses have evolved a number of strategies for evading the immune system to establish a chronic infection. Some viruses encode specific genes that target infected cells or the immune system. Some viruses persist in certain cell types where they are hidden from the immune system. Some viruses limit their replication, thus limiting the antigen available to alert the immune system. Some viruses are error prone in their replication, increasing the probability of generation of escape mutants. These viral mechanisms of immune evasion are balanced by mechanisms of regulation within the immune system.

It is well established that there is a mechanism for inhibiting the immune response during chronic infection. This is done for two reasons. One reason is to prevent immunopathology. CD8+ T cell effector function can cause high levels of tissue damage through killing of infected cells and release of inflammatory cytokines. In fact, cytotoxicity and secretion of cytokines such as tumor necrosis factor (TNF) are often decreased if not lost in CD8+
T cells in a phenomenon known as T cell exhaustion (1). The second reason is to prevent excessive proliferation of virus specific T cells. During acute infection, virus specific T cells can increase 10 fold each day. This level of proliferation is dangerous and is therefore greatly reduced upon continued exposure to antigen.

T cell exhaustion, demonstrated through diminished cytotoxicity, decreased cytokine production, and T cell deletion has long been recognized to occur during chronic infection. The causative factors and their contribution to viral persistence have only recently begun to be elucidated. Understanding the pathways contributing to T cell exhaustion will aid in development of novel treatment strategies for chronically infected individuals (1).

Lymphocytic choriomeningitis virus (LCMV), an arenavirus, is common mouse model for the study of chronic viremia. The murine response to LCMV clone 13 parallels what is seen in human HIV, HCV, and HBV infection. A classical CD8+ T cell effector response is induced, but infection is not cleared. As time with increased viral load progresses, T cells lose the ability to exert their effector function.

IL-10 is an immunosuppressive cytokine that suppresses T cells and antigen presenting cells by inhibiting proinflammatory cytokine production, co-stimulation, major histocompatibility complex (MHC) class II expression and chemokine secretion. Increased levels of IL-10 have been associated with chronic infection with HCV, HIV, and Epstein Barr Virus. CD4+ and CD8+ T cells have been shown to express high levels of IL-10 in HIV infected individuals.

The role of IL-10 as an immunosuppressive cytokine made it an attractive candidate to evaluate for a role in T cell exhaustion in chronic infection. Persistent viral infection has been shown to increase production of IL-10 by antigen presenting cells (APCs). Genetic removal of IL-10 resulted in a loss of the T cell exhaustion phenotype in a LCMV chronic infection model. Cells maintained a robust effector function, virus was rapidly eliminated, and antiviral memory was developed. Blockade of signaling through the IL-10 receptor via antibody treatment resulted in a similar outcome. T cell effector function was restored and the viral infection was eliminated (2, 3).

Dendritic cells (DCs) are key regulators of the immune response and play a major role in clearance of viral infections. DCs determine whether the T cell response will be an IFN γ producing Th1 response or an IL-4 producing Th2 response. In LCMV infected mice, levels of Th1 inducing CD8α+ DCs declined, while CD8α- DCs were not affected. The CD8α- DCs supported IL-10 production and the resulting decrease in antiviral T cell response (3).

In an effort to understand the mechanism behind the exhausted T cell response, Barber et al compared gene expression in CD8+ T cells chronically infected with LCMV clone 13 with functional memory CD8+ T cells obtained from mice infected with LCMV Armstrong, a clone that causes acute infection that is cleared within one week (4). Programmed death-1 (PD-1) was found to be upregulated in exhausted T cells. PD-1 is an inhibitory receptor in the CD28 family that is involved in regulating the immune response to self antigen. Ligation of this receptor results in signaling that acts on the TCR signaling pathway, reducing TCR mediated T cell activation and cytokine production. Virus specific T cell responses were enhanced by blocking PD-1 interaction with its ligand. Cytokine secretion and virus clearance was improved in anti-PD-1 antibody treated cells compared to untreated controls (4).

Transforming growth factor beta (TGFβ) was more recently shown to contribute to deletion of virus specific T cells. TGFβ is involved in regulation of cell proliferation, differentiation, survival and adhesion and functions in the suppression of autoreactive T cells. Tinoco et al reported observation of sustained TGFβ expression and Smad2 phosphorylation in virus specific CD8+ T cells, resulting in TGFβ dependent apoptosis. This correlated with upregulation of the proapoptotic factor Bim. Inhibition of TGFβ signaling resulted in increased numbers and effector function in antiviral CD8+ T cells. This allowed for rapid clearance of virus and generation of memory (5).

Back to top IL-7 as a strategy for reinvigorating the antiviral response

It has been shown that a number of immunosuppressive mechanisms are involved in maintaining a state of persistent infection. Inhibition of IL-10, PD-1, and TGFβ results in a revitalized immune response and viral clearance. With over 50 million people worldwide suffering from chronic infections, there is cause for developing a strategy to boost the immune response while limiting immune mediated pathology.

IL-7 is a cytokine that functions in development and regulation of T cell homeostasis. Pellegrini et al hypothesized that since IL-7 was a nonredundant cytokine involved in homeostatic proliferation and development, it would be able to overcome intrinsic mechanisms to limit T cell expansion. Using LCMV clone 13, Pellegrini et al treated chronically infected mice with IL-7 and observed increased viral clearance and increased
T cell numbers and function with no detectible liver damage. They found that this response was dependent on intrinsic IL-6 production. In addition, IL-7 promoted production of IL-22 which mediated cytoprotective effects (6).

IL-7 appeared to downregulate SOCS3, a repressor of cytokine signaling. This resulted in increased levels of cytokine production which enhanced T cell numbers, function and thus viral clearance. These effects offer promise in the treatment of chronic viral infections.

There has been considerable progress in our understanding of the immune response during chronic infection and overcoming immune suppression, however a number of questions still remain. How does IL-7 treatment function in upregulation of IL-6 expression? It is still unclear how IL-6 signaling and SOCS3 deficiency function in increasing the antiviral T cell response. Pellegrini et al proposes that IL-7 induced Th17 cells may protect against viral infection, but these cells are more commonly associated with antibacterial and antifungal immunity. Many questions remain about the role of Th17 cells in antiviral immunity.